1. Field of the Invention
The present invention relates to a prismatic battery that has a rectangular, round rectangular, or oval cross section and to its manufacturing method, and more particularly to an electrode terminal structure of the battery.
2. Description of the Related Art
Japanese Utility Model Laid-Open Publication No. 6-45253 shows a sealing structure of a battery that has an electrode pole as one terminal, and a battery case serving as the other terminal. The electrode pole is fixedly attached at its base end to a current collector that is welded to an electrode plate group, and fitted in a support cylinder formed in the lid of the case, with a sealing gasket inserted therebetween. The support cylinder is crimped to form an annular dent that has a square longitudinal cross section, so as to compress the gasket between the pole and cylinder for providing a seal.
Japanese Patent Laid-Open Publication No. 8-315790 shows a laser welding method for a prismatic battery wherein a lid is placed on the open end edge of a case and welded for sealing. As shown in FIG. 14, the outer periphery of the lid 52 is abutted to the inner periphery of the open end of the case 51, and the abutment surface is fused together to form a weld 53. The lid is first provisionally welded at three or more locations of the entire circumference before it is permanently welded to the case over the entire periphery.
Japanese Patent Laid-Open Publication No. 7-272701 shows a laser welding method for preventing battery packing defects resulting from packing damage caused by laser reflection light during the welding process. The outer surface of the lid, on which the packing is placed, is positioned higher than the open end of the case when projecting laser light, so that the fused part has a spherical surface downwards from the lid to the case. Thereby, the laser reflection light is directed opposite the packing.
Japanese Patent Laid-Open Publication No. 9-45296 shows a laser welding method for batteries wherein, as shown in FIG. 15, a lid 55 having a skirt 56 upwardly protruding from the outer periphery is fitted to the inner periphery of the open end of a cylindrical case 54, and the abutting edges of the open end of the case 54 and the skirt 56 are fused together to form a weld 57.
Battery case, whether it is cylindrical or prismatic, generally consists of a bottomed case accommodating an electrode plate group and a lid welded to the open end of the case, with or without a gasket interposed therebetween.
In such a manufacturing method of bottomed prismatic battery case, when a steel plate is used to manufacture the battery case, Japanese Patent Laid-Open Publication No. Hei 5-109393 shows a method for deep-drawing and pressing a nickel-plated steel plate with the nickel-plated side inside.
Japanese Patent Laid-Open Publication No. 9-237613 shows a prismatic case manufacturing method wherein a shallow bottomed cylindrical case that has been formed by pressing is drawn through several stages into a deep bottomed prismatic case.
Japanese Patent Laid-Open Publication No. 2003-208876 shows a method for forming a deep bottomed prismatic case by drawing and ironing processes, wherein a pellet is reformed into an intermediate cup having an oval cross section by impact forming, and the intermediate cup is then drawn and ironed continuously at one time.
Japanese Patent Laid-Open Publication No. 60-56376 shows a manufacturing method of a non-cylindrical battery wherein an electrode plate group wound around into a cylindrical shape is accommodated in a cylindrical outer case, and the case is pressed into a predetermined shape such as a prismatic shape, and closed with conforming lids at its both open ends.
With the technique shown in Japanese Utility-Model Laid-Open Publication No. 6-45253, the electrode pole is formed with a relatively deep annular dent having a square longitudinal cross section to a deepness approximately equal to the thickness of the sealing gasket. The support cylinder is plastically deformed (crimped) from outside so as to bend and press the gasket into the dent with a square longitudinal cross section, to provide a seal between the electrode pole and the support cylinder. With this method, there was the risk that cracks were generated in the gasket when crimping the support cylinder, leading to unreliable sealing properties. Also, the bent portion of the gasket was liable to break due to an impact external force.
Another problem was that, since the electrode pole, which was rigidly attached to the current collector, was fitted into the support cylinder formed on the lid, it was hard to achieve concentricity between the electrode pole and the support cylinder because of dimensional tolerances of the current collector and the electrode pole and dimensional errors of the case and lid. When the electrode pole and the support cylinder are not concentric with each other, the pressure is applied unevenly to the gasket when crimping the cylinder, whereby an adequate seal can not be formed because of insufficient compression of the gasket or because of breakage of the gasket caused by excessive compression.
The problem with the laser welding processes for prismatic battery cases shown in Japanese Patent Laid-Open Publications No. 8-315790 or No. 7-272701 was that, while the thick flat lid 52 was suitable for forming a strong weld 53 without impairing the flatness of the case 51 end face or the lid 52, its weight took up much of the battery weight because of its large thickness that was for withstanding pressure buildup to a predetermined level.
The lid 55 used in the laser welding method for sealing a battery shown in Japanese Patent Laid-Open Publication No. 9-45296 is thinner and more lightweight because of its structure wherein the skirt 56 is formed at the outer periphery. However, since, as shown in FIG. 16A, laser light 58 is projected vertically to the abutting edges of the case 54 end and the skirt 56 end, the abutting edges tend to fuse together entirely as shown in FIG. 16B, forming an uneven weld 59. This not only impairs the flatness of the case end face, but also may cause electrolyte leakage because of weld failure.
The problem could be solved by reducing the diameter of the laser beam 58 when projecting the beam to the abutting edge as shown in FIG. 16C, but in order to achieve required weld strength with such a small-diameter laser beam, it would be necessary to form a deep weld 60, which would take much time and reduce the productivity.
Prismatic batteries are often required to be flat and high so as to achieve large capacity and high cooling efficiency. With the deep drawing techniques shown in Japanese Patent Laid-Open Publications Nos. 5-109393, 9-237613, and 2003-208876, it was hard to produce bottomed cases with a large height while maintaining necessary productivity. The technique shown in Japanese Patent Laid-Open Publication No. 60-56376 could not be applied to produce flat prismatic batteries.
Another problem with deep drawing methods was that it did not allow much freedom of design for battery capacities because it was difficult to produce bottomed cases of various heights in view of negative cost factors such as required production equipment and change-over time.